The present invention relates to a control apparatus of a power converter for converting direct current into AC phase voltages each having three or more levels of potential and more particularly to the suppression of a fluctuation in neutral point voltage.
When a load such as an induction motor is driven by a pulse width modulation inverter, it is desirable that higher harmonic components contained in an AC output voltage of the inverter be reduced.
As an inverter meeting this requirement, an inverter called a three-level inverter has been proposed in, for example, Japanese Patent un-examined publication No. JP-A-56-74088.
A problem specific to the three-level inverter is that voltage at a series connection junction (hereinafter referred to as a neutral point) between voltage dividing capacitors connected in series is caused to fluctuate by current flowing into and flowing out of the neutral point. A technique of suppressing this fluctuation is disclosed in Japanese Patent unexamined publication No. JP-A-2-261063.
The neutral point voltage fluctuation suppressing technique disclosed in the above publications suppresses the fluctuation in voltage at the neutral point in feed forward fashion by calculating, from the primary frequency and power-factor angle, an instantaneous level of a command which nullifies zero-phase-sequence voltage and applying the command to a voltage command to thereby suppress the fluctuation in voltage at the neutral point.
In an application to, for example, a control apparatus of an induction motor for electric vehicle drive, however, the microprocessor has to perform other various operations and cannot afford to carry out such a complicated operation as above at an instantaneous level.